1. Field of the Invention
The present invention relates to amplification of radio frequency (RF) signals in those bands of frequencies in which resonant circuits comprised of discrete inductive and capacitive elements are used, and more particularly, to a linear amplifier which achieves substantially constant efficiency across a designated operating range.
2. Description of Related Art
The advent of high definition television (HDTV) has provoked renewed interest in the efficient amplification of UHF signals. HDTV transmitting systems will require amplifiers capable of extremely high data rates on the order of twenty-five megabits per second. To support these high data rates, digital modulation techniques, such as four or six level vestigial sideband modulation or 16 or 32-state double sideband quadrature amplitude modulation (QAM) are proposed. These forms of modulation, when used in a channel of limited bandwidth (e.g., 6 MHz), result in signals which have high ratios of peak to average power. It is extremely difficult to amplify such signals both efficiently and faithfully, that is, with very low distortion of the modulation content as measured by the absence of high-order intermodulation products. Thus, RF linear amplifiers capable of providing these characteristics are very desirable.
Traditionally, klystrons were used as the high power amplifiers for most UHF transmitters. A klystron is a linear beam device having an electron beam which is passed through a plurality of cavities. An RF input signal velocity modulates the beam and causes it to become bunched. The bunched beam induces an RF current in the cavities, and energy can be extracted from the bunched beam as an amplified RF output signal. However, klystrons are very inefficient at output powers lower than the maximum for which they are designed since they operate at constant voltage and current, and their efficiency is proportional to the output power.
A known technique for increasing the efficiency of a klystron is the use of a multistage depressed collector (MSDC). The electrons of the velocity modulated beam have widely varying energy levels as they exit from the output cavity. By using a multiplicity of collector electrodes which are depressed to potentials below that of the device body (i.e., the potential corresponding to the original electron beam energy), the spent electrons of the beam can be collected at the minimum possible energy. The electrons may be considered analogous to balls having various velocities that might roll up a hill until they stop and then roll back into traps on either side of their upward path. By recovering most of the remaining kinetic energy of the spent electron beam in depressed stages, beam energy is not lost by conversion of the kinetic energy into heat, and higher operating efficiency can be achieved. Multistage depressed collectors are described in Kosmahl, Modern Multistage Depressed Collectorsxe2x80x94A Review, Proceedings of the IEEE, volume 70, page 1325 (1982).
The efficiency of MSDC klystrons averaged over the modulation cycle has been shown to be up to three times that of conventional klystrons. Since the voltage at which the electrons are collected is roughly proportional to the RF output voltage of the klystron and the beam current is constant, the efficiency of the MSDC klystron is proportional to the square root of the output power. Despite this improved efficiency, MSDC klystrons do not provide the linearity necessary for the proposed HDTV transmitting systems.
Another type of amplifier utilizes one or more grids disposed between a cathode and an anode to density modulate current drawn from the cathode. It is a common practice to differentiate between amplifiers which use a grid to density modulate the electron stream on the basis of their operating regime, and they are categorized as either Class A, B or C. In a Class A amplifier, the grid bias and alternating grid voltages are applied such that the cathode current flows continuously through the electrical cycle. In a Class B amplifier, the control grid is operated at close to cutoff such that cathode current flows only during approximately half of the electrical cycle. Class AB amplifiers are hybrids of Class A and Class B amplifiers in which grid bias and alternating grid voltages are such that the beam current flow appreciatively more than half but less than the entire electrical cycle. Class C amplifiers have the grid bias appreciably greater than cutoff so that cathode current flows for appreciably less than half of the electrical cycle.
At lower frequencies, Class B amplifiers using triodes or tetrodes have demonstrated an ability to produce power more efficiently than conventional klystrons. In these amplifiers, the RF output current varies linearly with the cathode current and the voltage is constant, so the efficiency again varies as the square root of the output power as it does in the MSDC klystron. Tetrode and triode Class B amplifiers are effective for very high frequency (VHF) operation.
The advantages of Class B operation can be extended to higher frequencies by using a device known as an inductive output tube. Inductive output tubes have the same efficiency as other Class B amplifiers due to the fact that the RF input signal applied to a control grid causes the electron beam current to vary roughly as the RF drive voltage. Since the RF current in the tube does not result from velocity modulation, the amplifier is additionally highly linear.
The original inductive output tube was developed by A. V. Haeff, and consisted of a tubular glass envelope containing a cathode, a control grid disposed in front of the cathode, an accelerating aperture electrode and a collecting electrode. A gap of a re-entrant cavity was disposed in part of the tubular glass envelope between the accelerating aperture electrode and the collecting electrode. The electron beam generated by the cathode passed through the gap when focused by a magnetic field. When the electron beam was density modulated by the application of an RF input signal to the control grid at a frequency equal to the resonant frequency of the cavity, the electron beam current induced an electromagnetic wave in the cavity which extracted energy from the electrons without intercepting the electrons. The inductive output tube had the advantage over earlier vacuum tubes in that the interaction gap of the cavity could be of small area and have a low capacitance suitable for high frequency operation, while the electrons could be collected on a much larger collector electrode which no longer needed to be part of the resonant circuit.
The original concept for the inductive output tube was later recognized as being advantageous for use as a linear amplifier for UHF television signals. A modernized inductive output tube is disclosed in U.S. Pat. No. 4,480,210 for GRIDDED ELECTRON POWER TUBE, which includes a highly convergent electron gun with a pyrolytic-graphite control grid and a large collector. Making the control grid of pyrolytic-graphite, a highly refractory material, permits a much higher current density than previously possible in the original Haeff inductive output tube. This updated tube became known as a xe2x80x9cklystrodexe2x80x9d since it combined features of conventional klystrons with those of tetrodes; the klystrode has the resonant output cavity of a klystron, and the four electrode configuration of the tetrode.
Despite widespread knowledge of MSDC klystron and IOT efficiency enhancing techniques, a combination of the benefits of the inductive output tube with the multistage depressed collector was not actively pursued. The common wisdom in the art was that any improvement in efficiency gained by combining these features would be only on the order of 10% to 15% at peak power levels, and thus would not be worth the additional investment to modify existing designs. See Gilmour, Microwave Tubes, pages 196-200 (Artech House 1986). Moreover, it was believed that collector depression would require an increase in the cathode to anode voltage for a given power output, and if too much depression was used in attempting to increase efficiency, a deterioration in picture quality due to returned electrons across the cavity gap would result. See Preist and Shrader, The Klystrodexe2x80x94An Unusual Transmitting Tube With Potential For UHF-TV, Proceedings of the IEEE, volume 70, page 1318 (1982).
The parent of this application, U.S. Pat. No. 5,650,751, demonstrated that the conventional teachings in the art failed to recognize that an inductive output tube having a multistage depressed collector can provide near-constant and high efficiency across a power range of an RF signal through careful selection of the electrode stage potentials of the collector electrodes. Thus, it would be desirable to apply these teachings to lower frequency RF amplifiers with resonant circuits comprised of discrete inductive and capacitive elements rather than resonant cavities in order to achieve the same near-constant and high operating efficiency across a power range of the RF signal that has been heretofore demonstrated for UHF frequency levels.
In accordance with the teachings of this invention, an linear amplifier is provided for amplifying an input signal having a high ratio of peak to average power. The linear amplifier combines the teachings of the inductive output tube with the multistage depressed collector klystron to obtain enhanced operating efficiency and linearity. In the invention, the linear amplifier uses lumped constant resonant circuit elements rather than a tuned cavity in order to extend the advantages of constant high-efficiency operation to lower frequencies at which the sheer size of a cavity resonator would preclude its use.
The linear amplifier comprises an electron gun assembly having a cathode and an anode spaced from the cathode. A relatively high voltage potential is applied between the anode and the cathode, and the cathode provides an electron beam in response to the relatively high voltage potential. A control grid is spaced between the cathode and the anode, and is coupled to an input port adapted to receive the input signal. The input signal causes the control grid to density modulate the beam. The control grid is also coupled to a bias voltage source to preclude transmission of the electron beam during the negative half cycle of the input signal. A plurality of collector stages are provided with a respective electric potential thereto ranging between a potential of the cathode and a potential of the anode to efficiently collect the electrons of the beam after passing the anode. A first one of the collector stages is spaced from the anode opposite from the control grid and is further coupled to an output port to provide an amplified output signal therefrom. The respective electric potentials of the collector stages have corresponding voltage values such as to provide near-constant and high efficiency across a power range of the input signal.
More particularly, the electric potential applied to each respective collector stage is adjusted to preclude collection of the electrons at the relatively high voltage potential. The collector stages are coupled together by capacitors to preclude RF fields from forming therebetween, and the collector stages are coupled to the respective electric potentials through respective inductors. This way, electrons which have given up energy to the output grid will not be reaccelerated or further slowed by the collector stage elements.
A more complete understanding of the low frequency linear amplifier will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will be first described briefly.